1. Field of the Invention
The present invention relates to a semiconductor memory device provided with a MOS transistor having a charge accumulation layer and a control gate and a data write method of NAND flash memory, for example, a method for preventing erroneous write of the NAND flash memory.
2. Description of the Related Art
Conventionally, the NAND flash memory is well known as a nonvolatile semiconductor memory. A self-boost technique is widely used in the NAND flash memory. In the self-boost technique, a channel potential in write inhibit cell is increased by coupling to a gate, thereby preventing injection of electrons into a floating gate.
In the self-boost technique, it is known that boost efficiency is lowered by use of a memory cell in which data is written to increase a threshold. Therefore, there has been proposed a technique in which a channel of a memory cell on a source line side from a selected memory cell is cut off, thereby improving the boost efficiency with only an erased cell. For example, Jpn. Pat. Appln. KOKAI Publication No. 10-283788 discloses the technique.
However, in the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-283788, the channel cannot be cut off depending on the threshold voltage of the memory cell, lowering the boost efficiency. As a result, there is generated a problem in that the data is erroneously written to the write inhibit cell.
Recently, a multi-level technique is actively developed in the NAND flash memory. In the multi-level NAND flash memory, a threshold voltage range of each data is narrowed. As a result, a retention margin of the memory cell is decreased. In order to solve the problem, it can be considered that the threshold voltage of the memory cell is expanded toward a high-voltage side or a low-voltage side as a whole. However, when the threshold voltage is expanded toward the high-voltage side, it is necessary to increase a write voltage and a read voltage, which easily causes the erroneous write and erroneous read. When the threshold voltage is expanded toward the low-voltage side, the problem that the channel cannot be cut off in performing the self-boost becomes prominent.